Hydraulic Ciruits

 The function and sequence of individual components within a system are represented on a circuit diagram by means of symbols.

All hydraulic devices and connections must be identifiable on the diagram.

The circuit diagram together with the function diagram are indispensable for assembling a system and for later fault finding.

Circuit Diagram

Circuit diagram
1 Pump

2 Tank

3 Non return valve

4 Pressure unloading valve

5 Cylinder

6 Directional valve

7 Flow control valve.


*Closed Circuit

Circuit, closed

In the closed circuit the operating fluid flowing back from the hydraulic motor flows directly

to the hydraulic pump.

In order to fill the circuit and to continuously compensate for inevitable leakage.

a feed pump is used with a delivery which must normally amount to approx. 15 % of the main flow.

Separately adjustable pressure limiting valves protect the system against overloading.

*Open Circuit.

Circuit, open

With an open circuit the operating fluid flows from the tank to the hydraulic pump and is then transported to the

hydraulic motor.

From the hydraulic motor the medium flows unpressurized back to the tank, to be carried once more to the hydraulic pump.

The output direction of the hydraulic motor can be changed by means of a directional control valve.

A pressure limiting valve protects the system against overloading. The operating fluid is filtered on return.


Circuit, half open.

Circuit, half open

With a half-open circuit an insufficient volume flows in a certain flow direction from

the hydraulic motor back to the hydraulic pump.

The remaining amount is sucked out of the tank by means of an anti-cavitation valve.

The output device is normally a single rod cylinder, so that in the other flow direction

more operating fluid is directed to the hydraulic pump than it can take.

The difference is directed by means of a directional control valve to the tank.

Two pressure limiting valves protect the system against overloading.

Emergency actuation

Emergency actuation

In emergencies, e.g. during a power cut, (in addition to normal hand operation)

a working or closing stroke is carried out with the help of the energy in the accumulator.

In an emergency the valve reacts and the oil available in the accumulator is fed to the piston rod side of the

cylinder. This causes the piston to retract.


by means of 2-way flow control valve

Meter-in by means of 2-way flow control valve

Here flow control valve (1) is placed in the pressure line between hydraulic pump (4) and actuator (2). This type of

control is recommended for hydraulic systems in which the actuator acts as a positive resistance (opposing force)to the controlled flow.



control by means of 2-way flow control valve

Meter-out control by means of 2-way flow control valve

Here, the flow control valve (1) is situated in the line between actuator (2) and the tank. This type of control is

recommended for hydraulic systems with negative (pulling) working loads, which tend to cause the cylinder piston

to move more quickly than the speed which corresponds to the delivery flow of the pump (4). The pressure unloading valve (3) must be set according to the highest actuator pressure.

Pressure loss test

Pressure loss test

With this test to ISO 3968 the pressure lost from filter housing equipment and filter elements is determined with relation to flow and viscosity.

By means of a variable pump installed in the test circuit the volume of flow may be changed as required.

Hydraulic oil of viscosity class ISO VG 32 is normally used as the test fluid.

Pressure loss through the housing and filter element is recorded. Measuring points p1 and p2 must be upstream of the test filter: 5 x DI

Downstream of the test filter: 10 x DI

on a straight piece of piping. (DI = internal diameter of the piping).